The present invention relates to an LED driving apparatus and a method of controlling luminous power.
A display, such as a liquid crystal display, that is not a self-emitting display is provided with a backlight. As such a backlight, one employing cold-cathode tubes as its light source has been known for example. In addition, in recent years, backlights employing light emitting diodes (LEDs), which consume less power, have also been known.
As a method of controlling the luminous power of such a backlight including LEDs, two kinds of methods have been known. One is a current value control method for controlling the level of a driving current supplied to the LEDs. Another is to implement pulse width modulation (PWM) control with keeping a driving current level constant.
In the current value control, the level itself of a driving current continuously supplied to the LEDs is changed to thereby achieve aimed luminous power. In the PWM control, the ON/OFF ratio of a driving current per unit time is changed to thereby achieve desired luminous power.
An example of documents disclosing the related art is Japanese Patent Laid-open No. 4-134486.
It has been known that the luminous efficiency of an LED varies depending on the value of a driving current therefor.
This respect will be described with reference to FIGS. 7 to 9.
FIG. 7 illustrates a graph of the relationship between forward voltages and forward currents of a certain LED.
FIG. 8 illustrates a graph of the relationship between the forward currents and luminous power of the LED. Specifically,
FIG. 7 shows the respective values of the forward voltages obtained when forward currents of certain values are applied to the LED. FIG. 8 shows the respective values of the luminous power obtained when the forward currents of the certain values are applied to the LED.
Luminous efficiency is obtained by dividing luminous power by input power. Therefore, the luminous efficiency of the LED is obtained through the following procedure: a certain forward current value in the graph of FIG. 7 is multiplied by the corresponding forward voltage value to obtain the input power; and the luminous power value in FIG. 8 corresponding to the forward current value is divided by the obtained input power.
FIG. 9 illustrates a graph of the relationship between luminous efficiency that can be obtained through the above procedure and the forward current values.
As the graph shows, in the LED, the luminous efficiency increases as the forward current value increases from 50 mA to 100 mA, and the luminous efficiency decreases as the forward current value increases above 100 mA. The maximum luminous efficiency is obtained near a current value of 100 mA.
As is apparent from this graph, LEDs have characteristics in that the luminous efficiency varies depending on the value of the forward current (driving current). Specifically, the luminous efficiency of an LED is apt to increase as the driving current value increases until a certain current level, while above this level, the luminous efficiency is apt to decrease as the current value increases.
Since the luminous efficiency of an LED varies depending on the driving current value, the following problems arise in an LED driving apparatus that controls the luminous efficiency of an LED with any of the above-described methods: current value control and PWM control.
In the current value control, if the target value of the luminous power to be controlled is identical to the value corresponding to the current value providing the highest luminous efficiency like that shown in FIG. 9, the LED can be driven with the highest luminous efficiency. However, the target value does not necessarily correspond with the value, and therefore there is a possibility that the LED is driven with a low luminous efficiency. with a low luminous efficiency.
As for the PWM control, ON/OFF of a current is controlled with keeping the value of the current constant. Therefore, the constant current value must be the value corresponding to the maximum luminous power value in the allowable range thereof.
The current value for the maximum luminous power in the allowable range also does not necessarily correspond with the above current value for the highest luminous efficiency. Accordingly, there is a possibility that the LED is driven with a low luminous efficiency also in the PWM control.
Driving an LED with a low luminous efficiency requires unnecessary extra power higher than originally needed input power, which leads to the increase of power consumption. In addition, if an LED is driven with a low luminous efficiency, the heating value of the LED, a driving circuit thereof, and a power supply unit is apt to increase, which causes a problem that measures against the heating preclude the miniaturization of the device, for example.